The major outer membrane protein (MOMP) of Chlamydia trachomatis has been identified as a possible immunogen for vaccine development. The objectives of this project are to identify amino acid sequences of MOMP which are surface accessible to immunoglobulins and to determine if these sequences are immunodominant during infection. Peptides containing sequences of the four variable domains (VDI, VDII, VDIII, and VDIV) have been synthesized. These peptides have been used to define the epitope binding site of MAbs to MOMP, in particular, of MAbs which are known to passively project against infection in the monkey eye or in the mouse toxicity test. The antibody response during infection to the VDs of C. psittaci strain GPIC will be tested using synthetic peptides of all four VDs of GPIC and testing sera and tears from infected and immune guinea pigs. Polyclonal rabbit antisera is being raised to VDIII of A, E, and L2 MOMP to determine if this VD is surface accessible and to examine the homology of VDIII among serovars. Trypsin cleavage of MOMP at the surface of viable EBs results in loss of binding of EBs to HeLa 229 cells. Loss of binding occurs only when B MOMP is cleaved in both VDII and VDIV. suggesting a role for MOMP in interactions of chlamydiae with host cells. Synthetic peptides corresponding to VDII and VDIV of B and L2 serovars have been synthesized and will be used to determine their role in binding of chlamydiae to HeLa 229 cells.